Process for the manufacture of piston rings



United States PatentQ PROCESS FOR THE MANUFACTURE or PISTON RINGS LucienPras, Billancourt, France, assignor to Regie Nationale des UsinesRenault, Billan'court, France No Drawing. Application June 30, 1955'Serial No. 519,220

Claims priority, application France August 4, 1954 The invention relatesto piston rings for internal combustion engines and relates to a processfor the manufacture of said-piston rings enabling a satisfactory sealingof the combustion chamber to be eifected and the wear on said rings tobe reduced.

The piston rings of explosion engines or internal combustion engines arecurrently manufactured by stack moulding in green sand from a cast iron,the average composition of which is: C=3.50%; Si=3.0%; Mn=0.60%;P=0.40%; S=0.05%.

The aim, in this usual manufacture, is for the cast metal to consist ofgraphite, pearlite and possiblysteadite (phosphorus eutectic Fe-C-P).Every elfort is made to avoid cementite, the presence of which wouldrender any machining impossible, and ferrite, a soft constituent, thepresence of which is a cause of wear.

The imperative requirements of machining lead to working rules in thefoundry which reliably avoid cementite, but in these circumstances it isdifiicult to avoid ferrite. Slight variations in the casting conditionscause noticeable variations in the proportion of the latter constituent,so that constant supervision is necessary to ensure constancy in thequality of such a product.

The piston rings thus produced have long proved satisfactory in use.They are becoming inadequate, however, even when the desired structureof fine graphite Patented July 21, 1959 carbon capable of beingdissolved at the temperature in question, to be so. A temperaturebetween 760 and 920C. is suitable. This heating is followed byquenching, preferably in oil, thus conferring a martensitic structure tothe metal. It is also possible to efiect a step quenching comprisinghomogenization at a temperature and pearlite is obtained, in the face ofthe-increased requirements of modern e ngine's' 'Which are subjected tor he loads ndrc atioaa zspe s. .1 1

When conventional piston rings are used in the most modern engines, ofcars for example, sealing defects and premature wear appear, which causea reduction in power and an excessive consumption of lubricating oil.

An important progress is-achieved by the present invention, whichrelates to a method of manufacturing piston rings which makes itpossible to achieve an excellentsealing of the combustion chamber, saidsealing beingmaintained during ,long use of the engine, so that the needfor dismantling never arises by reason of'deterioration of the pistonrings.

According to the invention, the piston rings are obtained by cast ironmoulding, the composition of the iron being comprised within thefollowing limits:

Te 0.000 to 0.050%

After casting, the parts, which are entirely white in structure, aresubjected to a treatment which comprises the following sequence ofoperations:

(a) Reheating to a temperature at which austenitization takes place andfor a time suflicient for all the very slightly above the beginning ofthe martensitic transformation and short enough in duration to preventthe beginning of any bainitic transformation, then subsequent cooling toproduce the martensitic transformation.

(b) A second heating, to bring forth nucleation of the graphite, at atemperature comprised between 420 and 550 C. for a time comprisedbetween 10 minutes and 3, hours.

(c) A third heating, up to a temperature greater than the temperature atthe end of the eutectoid transformation, and maintenance at thistemperature for the time sufiicient and necessary for the carbon in theprimary cementite to be entirely graphitized. This heating is followedby cooling, either in still air or in oil, depending on the finalhardness desired.

(d) Temperingat a suitable temperature to ensure the desired degree ofhardness. For example, when graphitization has been followed by oilquenching, the tempering will be eifected at:

600 C. for a desired final hardness of 105 RB 560 C. for a desired finalhardness of 35 RC 520 C. for a desired final hardness of 38 RC (e) Afterthese thermal operations, the piston rings are machined and split. Asuitable tension is conferred on them by relieving the stresses set upwhile the ends of the ring are suitably parted. The stress relief isobtained by heating at a temperature slightly less than the temperingtemperature or by any other equivalent means.

The piston rings thus obtained are characterized metallographically:

By the absence of primary cementite;

By a distribution of the temper carbon in very fine, very numerous andvery regular granules, e.g.' 6,000 granulesof graphite for mmP;

By a matrix having'a very fine sorbitic-or pearlitic structure;

By a modulus of elasticity greater by about 50% than that'of ordinarygrey cast iron. For example, the modulus of elasticity of the pistonrings accordingto the invention has been found to be 18,000 kg./m.m.whereas the modulus of the conventional piston rings of grey cast irondoes not exceed 13,000 kg./mm.

By ahigher tensile strength and hardness than those of theusualpiston-ring metals. For example, in the pistonrings according to theinvention, a tensile strength of kg./mm.'- and a breaking load ofkg./mm. are found;

By a remarkable absence of brittleness such as is not found in the usualpiston-ring metals;

By a very low coefiicient of friction.

A specific example of manufacture of piston rings according to theinvention is given below.

After stack casting in green sand, piston rings of cast iron having thecomposition:

0 Si Mn Cu P S Nucleation heating for 2 hours at 450 C.;

Graphitization heating for 1 hour at 890 6., thencooling in still air;

.Tempering at 600 C., thu s obtaining ahardness of from 103 to 104 RB(measured by a ball in the Rockwell machine, load 100 kg); 7

Machining, and keeping the ends at the required distance during heatingfor 5 to 7 minutes at'580 (5.;

Grinding to the final dimensions.

The piston rings manufactured as described above ensure a very longperiod of service. They are particu larly valuable in engines in whichthe cylinder linings are of hard cast iron. In such engines, the. wearof the new piston rings is much less (by about30%) than that of pistonrings of conventional manufactureunder the same conditions of use. Thebreakages in se viee, which occur fairly frequently in verycut-awayscraper-rings currently manufactured, never occur with the newpiston rings.

I claim:

1. The process of malging alloyed castpiston rings which aresubstantially free" of primary cefnentite and brittleness, and have alow coeflicient of friction, comprising stack molding in a green sandmold the piston rings of a cast iron having the composition 3.004.00.%carbon, 1.503.10% silicon, 0.6-1.5% manganese, 0.8-2.0% copper,0.05O.70% phosphorus, .02-0. l% sulphur, and the balance substantiallyall iron, whereby the cast piston rings have a white structure, heatingto a temperature slightly above that of the eutectoid trans formationpoint so that austenitization takes place and for a time suflicient forall the carbon to be dissolved, quenching while maintaining asubstantially martensitic structure, again heating at a constanttemperature. between 420 C. and 550 C. for a period of the order ofthree hours to bring forth graphite nucleation, inereasing thetemperature after said nucleation heating to a temperature higher thanthe eutectoid transformation point for a period of time sufiicient tographitize the carbon in the primary cementite, cooling at a controlledrate, said cooling rate being a function of the final hardness, to beobtained, and tempering at a predetermined temperature to insure apredetermined hardness.

2. The process of making alloyed cast piston rings which aresubstantially free of primary cementite and brittleness, and have a lowcoeflicient of frietion, comprising stack molding in a green sand moldthe piston r n of a cast ro h in th om si iq 3.004.00% carbon 0.054070%phosphorus 1.so-3.10% silicon 0.024115% sulphur 0.6-1.5% manganese00.05% telluriu n 0.8-2.0% copper and the balance substantially alliron, whereby the east piston rings have a white structure, heating to atemperature slightly above that of the euteetoid transformation point sothat austenitization takes place and fear a time sufiicient for all thecarbon to be dissolved, quenchingin oil While maintaining asubstantially martensitic structure, again heating at a constanttemperature between 420 C. and 550 C. for a period of the order of 3hours to bring forth graphite nucleation, increasing the temperatureafter said nucleoli germination heating to a temperature higher. thanthe eutectoid transformation point for a period of time sufiicient tototally graphitize the carbon in the primary cementite, cooling at acontrolled rate, said cooling rate being a function of the finalhardness to be obtained, tempering at a predetermined temperature toensure a predetermined hardness, machining the rings and cutting a gaptherein to split the rings, stress relieving the rings at apredetermined temperature with the split being set at a predeterminedgap opening, said stgess relieving temperature being slightly less thanthe tempering temperature.

3. Th Process of making al oyed ast piston r n whisk: are bs n iall firs 9! rima y cemen t an bri tl n ss. an ha a l w oefli s t of i tion,inpri ins stack meldin in a e n nd mold h pis on rings of a cast ironhaving the composition 3.54% carbon 1.86% silicon 1.24% manganese 1.29%copper 0.44% phosphorus 0.04% sulphur and the balance substantially alliron, whereby the cast piston rings have a white structure, heating to atemperature of 810 C. and maintaining this temperature for substantially30 minutes so that austenitization takes place and for all the carbon tobe dissolved, quenching in oil while maintaining a substantiallymarteiisitic stiucture, again heating at a constant temperature of 450C. for a period of two hours to enhance graphite nucleation,graphitization heating at a temperature of 890 C. for 1 hour, cooling instill air at a predetermined rate necessary to obtain a predeterminedhardness, tempering at substant ally 600 C. for a period of time toobtain a hardness of between 103 and 104 RB when measured by a ball typeRockwell machine with a load of kg, machining the rings and cutting agap therein to split the rings, stress relieving the rings at atemperature of substantially 580 for a period of between 5 to 7 minuteswith the split in the rings being set at a predetermined gap opening,and grinding to final dimensions.

Refie -ences Cited in the tile of this patent S E A E S 2. 85. 9 Hul srn ---..-7--.-..-- a 2, 1 40 ,650 B nc f -=1 Sep 0. 0 2331.88 i g hol t-1 1943

1. THE PROCESS OF MAKING ALLOYED CAST PISTON RINGS WHICH ARESUBSTANTIALLY FREE OF PRIMARY CEMENTITE AND STACK MOLDING IN A GREENSAND MOLD THE PISTON RINGS OF A CAST IRON HAVING THE COMPOSITION3.00-4.00% CARBON 1.50-3.10%SILICON,0.6-15% MANGANESE, 0.8-2.0%COPPER,0.05-0.70% PHHOSPHOURS,2.02-0.15% SULPHUR,AND THE BALANCE SUBSTANTAILLYALL IRON WHEREBY THE CAST PISTON RING HAVE A WHITE STRUCTURE HEATING TOA TEMPERATURE SLIGHTLY ABOVE THAT OF THE EUTECTOID TANSFORMATION POINTSO THAT AUSTENITIZATION TAKES PLACE AND FOR A TIME SUFFICIENT FOR ALLTHE CARBON TO BE DISSOLVED QUENCHING WHILE MAINTAINING A SUBSTANSTIALLYMARTENSITIC STRUCTURE AGAIN HEATING AT A CONSTANT TEMPERATURE BETWEEN420*C.AND 550*C. FOR A PERIOD OF THE ORDER OF THREE HOUR TO BRING FORTHGRAPHITE NUCLEATION INCREASING THE TEMPERATURE AFTER SAID NUCLEATIONHEATING TO A TEMPERATURE TURE HIGHER THAN THE EUTECTOID TRANSFORMATIONPOINT FOR A PERIOD OF TIME SUFFICENT TO GRAPHITZE THE CARBON IN THEPRIMARY CEMENTITE COOLING AT A CONTROLLED RATE SAID COOLING RATE BEINGTEMPERING AT A PREDETERMINED TEMPERATURE TO INSURE A PREDETERMINEDHARDNESS.